Chapter Notes 1 - Chapter 5 : Acids, Bases and Salts, Class 7, Science | EduRev Notes

Chapter Notes - Acids, Bases and Salts

Indicators

Many substances that you encounter in your daily life have different properties. One major difference between substances is whether they are acidic or basic.

Many substances that you encounter in your daily life have different properties. One major difference between substances is whether they are acidic or basic. Acidic substances taste sour. Some examples are curd, lemon juice, tamarind, spinach and vinegar. These contain natural acids. Basic substances taste bitter and feel slippery to the touch. Some examples are baking soda and milk of magnesia. There are special substances that are used to test whether something is acidic or basic. These substances are called indicators. Indicators change their colour when added to an acidic or basic solution. Some natural indicators are litmus, turmeric and china rose petals.

Litmus is a natural dye extracted from lichens. It is the most commonly used natural indicator. A solution of litmus turns red if it is acidic, and blue if it is basic.

Turmeric changes colour to indicate acidic and basic substances. It is yellow in acids and reddish brown in basic solutions.

China rose is also a natural indicator. A solution of china rose turns green in a basic solution, and bright pink or magenta in an acidic solution.

Acid rain is rain that contains a lot of acid. This acid is created when pollutants in the air, such as carbon dioxide, sulphur dioxide and nitrogen dioxide, dissolve in rainwater, and form acids like carbonic acid, sulphuric acid and nitric acid
This rain causes a lot of damage to buildings, plants and animals. This is one important reason why we must avoid polluting our atmosphere.
There are certain substances that indicate the acidic or basic nature of other substances. Such special substances are called indicators. An indicator changes its colour to show the nature of a substance.

Some natural indicators are turmeric, litmus and China rose.

Litmus is a common natural indicator extracted from lichens.

Litmus has a purple colour in a neutral medium, red colour in an acidic medium, and blue in a basic medium.

When a drop of freshly prepared lemon juice is put on red litmus, it remains red, indicating the acidic nature of lemon juice.

If blue litmus is used instead, then it turns red, again indicating the acidic nature of lemon juice.

Turmeric paper changes colour in a basic medium.

In an acidic medium, it remains yellow, while in a basic medium, it turns reddish brown.

When a drop of soap solution is put on turmeric paper, it turns red. This indicates the basic nature of soap solution.

China rose is another indicator extracted from China rose petals. In an acidic medium, it exhibits dark pink (magenta), while in a basic medium, it exhibits green colour.

Neutralisation

Substances have either acidic, basic or neutral properties.

Substances have either acidic, basic or neutral properties. Acids are sour to the taste, and include substances like curd, vinegar, lemon, orange juice, etc.

Bases are bitter to the taste and slippery to the touch, and include substances like baking soda milk of magnesia, etc.
When an acid is mixed with a base in the right proportion, they neutralise the effect of each other. That means the acidic nature of the acid and the basic nature of the base are destroyed. This reaction is called neutralisation. The neutralisation reaction between an acid and a base can be identified by an indicator. For example, for the reaction between hydrochloric acid and sodium hydroxide, phenolphthalein is added as an indicator. Phenolphthalein is pink in sodium hydroxide. When hydrochloric acid is added to it, the colour slowly fades to pale pink and then colourless. This change in colour indicates neutralisation.

The neutralisation reaction between hydrochloric acid and sodium hydroxide can be written as

Hydrochloric acid +Sodium hydroxide → Sodium chloride + Water Heat 

is also produced during a neutralisation reaction. Neutralisation is useful in our daily life.

Acidity caused by indigestion can be reduced by neutralising the hydrochloric acid in the stomach by a base, like milk of magnesia.

An ant's sting gives a lot of pain due to formic acid. It can be neutralised by applying calamine lotion, or rubbing with baking soda.
Excessive use of chemical fertilisers makes soil acidic. Neutralising the soil with a base like quick lime, which is calcium oxide, or slaked lime, which is calcium hydroxide, will make this soil suitable for plants.

Neutralisation is the reaction between an acid and a base to form salt and water. During neutralisation, salt and water are formed, along with the release of some heat.

Eg: HCl + NaOH------------>NaCl + H₂O + Heat

Eg: H₂SO₄ + 2NaOH--------------->Na₂SO₄ + 2H₂O + Heat

The stomach contains hydrochloric acid, which helps in the digestion of food. However, when this acid is produced in excess, it causes indigestion, which is painful. An antacid such as milk of magnesia neutralises the excessive acid in the stomach, and provides relief from the pain due to indigestion.

Ant sting contains formic acid. When an ant bites, it injects this formic acid into the skin. This causes pain. It can be neutralised by rubbing the ant bite with moist baking soda, which contains sodium hydrogen carbonate (NaHCO3), or with a solution of calamine, which contains zinc carbonate (ZnCO₃).
Excessive use of fertilisers makes soil acidic. As a result, plants cannot grow properly, and the yield decreases. Adding a base like quick lime, (calcium oxide) or slaked lime (calcium hydroxide) neutralises the soil and makes it suitable for plants.
Similarly, when soil is basic in nature, organic matter is added to release acid and make it neutral, and thus, suitable for plants.
Factory waste contains acids. This waste must be treated with bases for neutralisation before it is released into a water source. Otherwise, it can cause damage to living organisms in the water source.

Important Points:

• Acid is sour in taste.
• Acid turns blue litmus paper to red.
• Acid turns China rose solution to dark pink.
• Base is bitter in taste.
• Base turns red litmus paper to blue.
• Base turns turmeric paper or solution to red.
• Base turns China rose solution into green.
• Indicator is a special substance which shows the acidic or basic character of a substance by change in colour.
• Litmus, turmeric and solution of China rose are natural indicators.
• Litmus is obtained from lichens.
• Solutions of acid and base neutralize each other when mixed in proper ratio. This is called neutralization or neutralization reaction.
• In neutralization reaction between the solutions of acid and base, a new substance called salt is formed.
• Salt formed because of neutralization reaction may be acidic or basic in nature. The acidic or basic nature of salt depends upon the strength of acid and base used for neutralization reaction.